Clarify PS-part of struct vlr_subscr

There's struct { struct llist_head pdp_list; } ps; in struct vlr_subscr which is commented as related to SGSN. Clarify if it's actually necessary or it's just leftover from pre-split repos. If former than comment should be added explaining how/what for it's used otherwise it (and functions using it) should be removed.

History

osmo-sgsn is using its own GSUP integration towards HLR, the PS part of libvlr in osmo-msc has no user and has never been used.Of course the initial idea is to use the same libvlr for both osmo-msc and osmo-sgsn, but it's not super likely to happen.